1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium to be installed in any of various magnetic recording apparatuses.
2. Description of the Related Art
Since 1997, the recording density of hard disk drives (HDDs) has increased rapidly at a rate of 60 to 100% per year. As a result of this remarkable growth, a limit is being approached for increasing the recording density with hitherto used longitudinal recording. Due to this state of affairs, in recent years, a perpendicular recording method for which the recording density can be increased has come to receive attention, and research and development have been carried out with vigor. In 2005, commercialization of HDDs using the perpendicular recording method finally started for some HDD models.
A perpendicular magnetic recording medium comprises a magnetic recording layer made of a hard magnetic material, and an underlayer made of a soft magnetic material that fulfils the role of concentrating magnetic flux produced by a magnetic head used when recording onto the recording layer. Of these layers, the soft underlayer is sometimes regarded as part of the head, and is an essential component for maintaining good writeability for the perpendicular magnetic recording medium.
However, problems with use of the soft underlayer have been identified including (1) to (3) which follow. (1) Noise known as spike noise results when magnetic domain walls form in the soft underlayer arises. (2) Noise additionally results due to the mirror-image of the magnetic recording layer being transferred to the soft underlayer. (3) Recorded magnetization may be lost if a stray magnetic field in the HDD or a magnetic field produced by the mirror-image written onto the magnetic recording layer forms a magnetic circuit with the soft underlayer via the head.
To suppress problem (1), hitherto there have been proposed, for example, a method in which a hard magnetic pinning layer is provided between the soft underlayer and the substrate, so as to orient the magnetization of the soft underlayer in one direction (see Japanese Patent Application Laid-open No. 7-129946), a method in which the magnetization is pinned by utilizing exchange coupling between the soft underlayer and an antiferromagnetic layer (see Japanese Patent Application Laid-open No. 6-103553), and a method in which two soft underlayers are provided and are antiferromagnetically coupled together (see Japanese Patent Application Laid-open No. 2001-155321).
However, with pinning using a hard magnetic pinning layer, it is very difficult to orient the magnetization of the soft underlayer in a specified direction (e.g., an outward radial direction) over the whole disk, and it is known that in actual practice spike noise caused by magnetic domain walls arises at inside and outside diameters. Moreover, to obtain a sufficient exchange coupling field with the method using an antiferromagnetic layer, a complicated and costly method has had to be used, for example, a heat treatment taking from several minutes to several hours must be carried out after film formation, or a multilayer lamination of soft magnetic layers and antiferromagnetic layers must be formed (see K. W. Wierman, et al., IEEE Trans. Magn., Vol. 37, No. 6, pp. 3956-3959, 2001). With the method utilizing two soft magnetic layers which are antiferromagnetically coupled together, a multidomain structure is formed in practice, and hence it has been found that spike noise arises, albeit only to a small extent.
Moreover, for problems (2) and (3), the current state of affairs is that these are regarded as problems, but thorough countermeasures have not been found as yet.
It is an object of the present invention to realize an improvement in medium performance by simultaneously resolving the following three problems for the soft underlayer: (1) the problem of noise known as spike noise caused by magnetic domain walls arising, (2) the problem of noise arising due to the mirror image of the magnetic recording layer being transferred, and (3) the problem of a stray magnetic field in the HDD or a magnetic field produced by magnetization written onto the magnetic recording layer forming a magnetic circuit with the soft underlayer via the head, whereby the recorded magnetization is lost.
To resolve these problems caused by the soft underlayer, the underlying mechanisms thereof must be considered.
For problem (1), it has been known for a long time that this is caused by magnetic domain walls formed in the soft underlayer, and various counter-measures have been devised as described above.
Problem (2) will be explained using a diagram. FIG. 1 consists of schematic views of the magnetization (shown by arrows in the diagram) of a magnetic recording layer 106 and a soft underlayer 102 (an upper portion only being visible in this view) of a conventional perpendicular magnetic recording medium. FIG. 1A shows the case of low-density recording. FIG. 1B shows the case of high-density recording. Note that for simplicity, only the magnetic recording layer and the soft underlayer are shown in FIG. 1, a nonmagnetic intermediate layer and so on being omitted. As shown in FIGS. 1A and 1B, the magnetization of the magnetic recording layer 106 is transferred to the soft underlayer 102. As shown for the case of low recording density in FIG. 1A, noise is caused merely through the magnetization of the soft underlayer 102 being oriented in a perpendicular direction. In addition, the recording resolution for the soft underlayer 102 is low, and hence for high-density recording as shown in FIG. 1B, the direction of the magnetization of the magnetic recording layer 106 and the direction of the magnetization of the soft underlayer 102 do not necessarily coincide, and hence noise is further increased. This is the mechanism by which noise arises for problem (2).
For problem (3), the problem that a stray magnetic field in the HDD may be concentrated in the head is known as an antenna effect. Moreover, a phenomenon in which magnetic flux produced by the magnetic recording layer is concentrated in a return yoke (or a trailing shield) of the head and this returns and erases the recorded data on the medium on the write pole side is known to be one cause of pole erasure (PE) (see A. Chekanov, E. N. Abarra, and G. Choe, Digests of the INTERMAG 2005, CB11, p. 255 (2005)). For the antenna effect and PE, improvements have been attempted predominantly on the head side, but the current state of affairs is that methods for improvement on the recording medium side have rarely if ever been proposed.
All of these problems occur due to the magnetization of an upper portion of the soft underlayer (the portion near the magnetic recording layer) being oriented in a perpendicular direction relative to the surface of the nonmagnetic substrate. It is thought that this phenomenon can be resolved by strongly orienting the magnetization of the soft underlayer in-plane, that is, parallel to the surface of the nonmagnetic substrate. However, if the magnetization is strongly oriented in-plane, then the effect of the soft underlayer drawing in the recording magnetic field produced by the head is weakened, and hence writeability suffers. It is thus thought that the state of “acting as a soft underlayer during recording, but the magnetization of an upper portion of the soft underlayer appearing to disappear during reading” is ideal, but such a structure has not previously been realized.